A particle beam accelerator accelerates electrically charged particles in vacuum. A cyclotron, one of the particle beam accelerators, accelerates them in a constant magnetic field with an alternating high frequency electric field generated between a pair of electrodes. Charged particles introduced from an ion source are accelerated to move along a spiral orbit with the period of the high frequency electric field. A particle beam moving along a circular orbit at the maximum radius is extracted towards the external to strike a target.
Particle beam accelerators such as cyclotrons are used in various fields. Compact cyclotrons are used in hospitals or the like in order to generate radioisotopes used for examination. For example, 15O nuclei are produced by irradiating 14N2 gas with a deuteron beam generated by a particle beam accelerator, and a drug is synthesized by a chemical reaction by using the radioisotopes. In such a system, a drug such as C15O gas is generated. As another example, a substance for cancer diagnosis is synthesized by using 18F generated with 18O(p, n)18F reaction.
As to a cyclotron, there is the principle that a momentum of an accelerated particle is proportional to a product of radius of curvature of the accelerated orbit and magnetic flux density. Therefore, if the magnetic flux density is constant, the size of a cyclotron becomes larger as the energy of the beam to be extracted becomes higher.
When the beam strikes a target thick enough to be stopped within the target, the number of isotopes generated by the nuclear reaction per unit current becomes larger as the energy of the beam becomes larger. Therefore, a deuteron beam is accelerated up to a relatively high energy of about 10 MeV in many cyclotrons used for drug synthesis.
On the other hand, for example, in a reaction for generating 15O from 14N, a sufficient amount of the drug can be synthesized with a deuteron beam of acceleration energy of about 3.5 MeV. For example, when the acceleration energy is 3.5 MeV, 15O label can be produced with a deuteron beam of about 500 mCi. Then, cyclotrons of a relatively small size are developed (for example, refer to Oxygen Generator System Product Description (Ion Beam Accelerations)).
Radioactive rays are generated when an energy beam from the particle beam accelerator injected directly or after scattering onto a substance. Generally, the accelerated particles strike not only the target, but also electrodes, inner walls, residual gas and a target cell in the accelerator. If particles scattered after striking the electrodes or the like have a sufficiently high energy, they may strike another component to generate radioactive rays. For example, in the above-mentioned reaction to radiate a deuteron beam onto 14N nuclei to generate 15O nuclei, neutrons and gamma rays may be generated. Further, other reaction processes also occur, so that various types of radioactive rays are generated in the accelerators.
Because radioactive rays affect a human body, it is important to decrease the amount of the generated radioactive rays. Therefore, a particle beam accelerator has various shields. Especially, neutrons and gamma rays are difficult to be shielded because they have high transparency against a substance, in contrast to charged particles. Then, an accelerator is set in a room having walls and a floor made of thick concrete.
However, a particle beam accelerator occupies a large volume and has a high weight, so that it is necessary to take the strength of the setting area into account sufficiently. Therefore, it is desirable to decrease the volume occupied by the accelerator and to reduce the weight thereof. In order to solve the problem, a self-shield is developed to cover a cyclotron as one of the accelerators with a shield for the main body of the accelerator and for radioactive rays generated at the target. For example, a concrete wall as thick as one meter is used as a self-shielding wall. Though a cyclotron of Ion Beam Accelerations is compact, the outer size of the concrete used for shielding the cyclotron is about 4*2.8*3.4 m in an open state. Thus, it is difficult to install such a cyclotron newly in an existing building. Therefore, it is desirable to provide a particle beam accelerator reduced further in size and weight.